A “shift drum type” transmission is often conventionally used in a straddle-type vehicle, for example. A shift drum type transmission includes: a plurality of first gears provided on a main shaft; and a plurality of second gears provided on a drive shaft. The first and second gears include movable gears that are movable axially. The shift drum type transmission further includes: a shift fork that axially moves the movable gears; and a shift drum provided with a groove with which the shift fork engages. The shift drum is connected to a shift shaft via a gear change mechanism. Upon rotation of the shift shaft, the shift drum rotates. Upon rotation of the shift drum, the shift fork moves along the groove. With the movement of the shift fork, the movable gears move axially. As a result, a combination of the first and second gears which intermesh with each other and to which a torque is transmitted is changed, thus changing a transmission gear ratio between the main shaft and the drive shaft.
The gear change mechanism includes: a shift lever fixed to the shift shaft; and a rotation plate fixed to an end of the shift drum. The shift lever is arranged to be engageable with the rotation plate. Upon rotation of the shift lever with the rotation of the shift shaft, the rotation plate receives a force from the shift lever and thus rotates. The shift drum rotates with the rotation of the rotation plate. As a result, a speed change is performed.
In general, a transmission is disposed inside a crankcase of a power unit. JP 2010-236451 A discloses a power unit in which a gear change mechanism is disposed in a lower portion of an interior of a crankcase. More specifically, JP 2010-236451 A discloses that a main shaft and a shift drum are disposed in a lower portion of an interior of the power unit, thus lowering its center of gravity.